Compression seals are widely used to seal the void between adjacent structural members, such as concrete slabs in bridges, parking decks, and the like, while absorbing thermal expansion and contraction of the adjacent structures. The seal prevents debris and water from penetrating the joint and causing intrastructure damage, accomplishing this goal by designed collapse of the interior walls of the joint throughout its movement range. Additionally, when the seal is incorporated into a structure designed to accomodate pedestrian traffic, the upper surface of the seal must provide a large supported planar treadway throughout the collapse and expansion of the seal.
Typically, a compression seal is formed from neoprene or the like as an elastomeric extrusion of indeterminate length. The cross-sectional configuration of the seal determines the manner in which the seal collapses as it is subjected to lateral compressive forces. The characteristics which are of primary concern in the design of compression seals are movement ratng and height displacement.
A seal's movement rating is the distance by which a seal can be comprssed from 85% of its nominal width to its width at maximim compression. Under current ASTM standards, when an installed seal is at maximum expansion, that is, when the adjacent structural members are fully thermally contracted, the seal is still compressed to 85% of its nominal width. At maximum expansion, the seal must exert a pressure of at least 3 p.s.i. against the adjacent structural members. This pressure is necessary to maintain the seal in place in the joint between the adjacent structural members, and to prevent moisture and debris from penetrating between the seal and the structural members. In contrast, the seal's maximum closure is defined as the point at which the seal exerts a lateral pressure of 35 p.s.i. against the adjacent structural members. The distance between 85% of the seal's nominal width and its width at maximum closure is the seal's movement rating. It is a desirable characteristic of a compression seal to have a large movement rating, since a narrower seal can be employed to absorb the same amount of thermal expansion and contraction of the adjacent structural members.
However, movement rating is only one characteristic which must be considered. While the seal must collapse in such a controlled manner as to postpone the point of maximum closure, it must also collapse in such a manner as to minimize vertical elongation, or height displacement, of the seal. A compression seal which protrudes above the top surface of the adjacent structrual members as it is compressed can be damaged or dislodged by snowplows or the like and can provide an obstacle upon which pedestrians can trip. When used in structures intended for only vehicular traffic, this problem can be overcome by designing a compression seal which collapses downwardly, rather than protrudes upwardly, as the seal is compressed. However, where the structure is intended to support pedestrian traffic, a downwardly-collapsing seal would present a depression into which women's high heels can become lodged. Accordingly, for pedestrian applications, it is necessary for the compression seal to provide a substantially planar upper surface at all points during the expansion and compression of the seal.